Corrosion and wear resistant alloy

ABSTRACT

A powder metallurgy corrosion and wear resistant tool steel article, and alloy thereof. The article is manufactured by hot isostatic compaction of nitrogen atomized, prealloyed high-chromium, high-vanadium, high-niobium powder particles. The alloy is characterized by very high wear and corrosion resistance, making it particularly useful for use in the manufacture of components for advanced bearing designs as well as machinery parts exposed to severe abrasive wear and corrosion conditions, as encountered, for example, in the plastic injection molding industry and food industry.

FIELD OF THE INVENTION

The invention relates to powder metallurgy corrosion and wear resistanttool steel alloy article, manufactured by hot isostatic compaction ofnitrogen atomized, prealloyed high-chromium, high-vanadium, high-niobiumpowder particles. The alloy of the article of the invention ischaracterized by very high wear and corrosion resistance, making it inparticular useful as a material from which to make components foradvanced bearing designs as well as machinery parts exposed to severeabrasive wear and corrosion conditions such as those, among many others,in the plastic injection molding industry and the food industry.

BACKGROUND OF THE INVENTION

To perform satisfactorily, the alloys that are used in a number ofdemanding applications—such as screws and barrels in the plasticinjection molding industry, for example—must be resistant to wear andcorrosion attack. The trend in the industry is to keep increasingprocessing parameters (e.g., temperature and pressure), which in turnimposes ever-increasing demands on the alloys and their ability tosuccessfully withstand corrosion attack and wear of the materials beingprocessed. In addition, the corrosiveness and abrasiveness of thosematerials are constantly increasing.

The wear resistance of tool steels depends on the amount, the type, andthe size distribution of primary carbides, as well as the overallhardness. The main function of primary alloy carbides, due to their veryhigh hardness, is to provide wear resistance. Of all types of primarycarbides commonly found in tool steels, V-rich and V—Nb-rich MC primarycarbides possess the highest hardness.

The corrosion resistance of tool steels depends primarily on the amountof “free” chromium in the matrix, i.e., the amount of chromium that isnot “tied up” into carbides. For good corrosion resistance,through-hardening tool steel must contain at least about 12 wt. % “free”chromium in the martensitic matrix after heat treatment. However,corrosion and wear resistant tool steels must also contain a relativelyhigh level of carbon for heat treatment response. As chromium has a highaffinity for carbon with which it forms chromium-rich carbides, acorrosion and wear resistant tool steel must contain excess chromium.

The corrosion resistance of tool steels is further improved by thepresence of molybdenum in the martensitic matrix. Some tool steels thatcontain about 10 wt. % “free” chromium in the martensitic matrix arecorrosion resistant because they also contain a sufficient amount of“free” molybdenum. An example is Crucible 154 CM grade, which is basedon the Fe-1.05C-14Cr-4Mo system.

In order to withstand the stresses imposed during operation, the toolsteel must also possess sufficient mechanical properties, such ashardness, bend fracture strength, and toughness. In addition, the toolsteel must possess sufficient hot workability, as well as machinabilityand grindability, to ensure that parts with the required shape anddimensions can be manufactured. In general, the higher the volumefraction of primary carbides, the higher the wear resistance of the toolsteel, and the lower its toughness and hot workability.

The corrosion and wear resistant martensitic tool steels currently usedinclude grades such as CPM S90V, M390, Elmax, Anval 10V-12, HTM X235,for example. Despite the fact that the overall chromium content of someof these alloys is as high as 20 wt. % (e.g., M390), the corrosionresistance is not necessarily as high as one might expect. Depending onthe overall chemical composition and the heat treatment parameters, alarge amount of chromium, which is a strong carbide former, is pulledout of the matrix and tied up into chromium-rich carbides. This tied upchromium does not contribute toward the corrosion resistance.

One of the practices that has been used to improve the combination ofresistance to corrosion and wear, as exemplified by CPM S90V, is to addvanadium. This alloying addition forms hard vanadium-rich MC primarycarbides and ties up a part of the carbon. Due to the fact that theaffinity of vanadium toward carbon is higher than that of chromium, thepresence of vanadium in tool steels decreases the amount ofchromium-rich primary carbides, all other conditions being equal (i.e.,the overall chromium and carbon content, the heat treatment parameters,for example). In the alloy of the invention, in addition to vanadium,niobium is used as well in order to further increase the amount of MCprimary carbides, and in turn decrease the amount of chromium-richprimary carbides, due to the fact that niobium has even a higheraffinity toward carbon than vanadium.

A primary object of the invention is to provide wear and corrosionresistant, high chromium, high vanadium, high niobium, powder metallurgytool steel article with significantly improved corrosion and wearresistance.

SUMMARY OF THE INVENTION

It has been discovered that the improved balance between wearresistance, the corrosion resistance, and the hardness of thehigh-chromium, high-vanadium, powder metallurgy martensitic stainlesssteel article of the invention is affected by adding niobium. The alloyarticle of the invention possesses a unique combination of corrosion andwear properties that are achieved by balancing its overall chemicalcomposition as well as selecting an appropriate heat treatment.

It has been discovered that the addition of niobium decreases thesolubility of chromium in V—Nb-rich MC primary carbides, which in turnincreases the amount of “free” chromium in the martensitic matrix.According to thermodynamic calculations, the carbon sublattice of theV—Nb-rich MC primary carbides that precipitate in the alloy of theinvention has less vacancies compared to the carbon sublattice of thecomparable V-rich MC primary carbides: (V, Nb)C_(0.83) versus VC_(0.79.)

It has been discovered that the presence of niobium in the alloy of theinvention also lowers the amount of chromium that dissolves in MCprimary carbides. This in turn increases the amount of “free” chromiumin the matrix, which further improves the corrosion resistance.

The major alloying elements used in the alloy of the invention(chromium, molybdenum, vanadium, and niobium) are ferrite stabilizers.High amounts of these ferrite stabilizers can lead to the presence offerrite in the heat-treated microstructure. It has been discovered,however, that the presence of about 2 wt. % cobalt in the alloyingsystem of the invention is a necessary and sufficient measure toeliminate ferrite in the heat-treated microstructure.

Finally, in order to obtain a desired combination of wear and corrosionresistance, along with good mechanical properties, such as bend fracturestrength, toughness, and grindability, it is necessary to controlclosely the atomization process (to obtain fine spherical powder) andthe hot isostatic parameters of the prealloyed powders as is well-knownin the art. The alloy of the invention is to be preferably hotisostatically pressed at the temperature of 2150° F. (±25° F.) and thepressure of at least 14.5 ksi.

In accordance with the invention, there is provided a corrosion and wearresistant article produced by hot isostatic composition of nitrogen gasatomized prealloyed powder particles within the following compositionlimits, in weight percent, carbon, 2.0 to 3.5, preferably 2.7 to 3.0;silicon 1.0 max.; chromium 12.0 to 16.0, preferably 13.5 to 14.5;molybdenum 2.0 to 5.0 preferably 3.0 to 4.0; vanadium 6.0 to 11.0,preferably 8.5 to 9.5; niobium 2.0 to 6.0, preferably 3.0 to 4.0; cobalt1.5 to 5.0, preferably 2.0 to 3.0; nitrogen 0.05 to 0.30, preferably0.10 to 0.20; and balance iron and incidental impurities.

Preferably, carbon is balanced with chromium, molybdenum, vanadium, andnitrogen in accordance withC _(min)=0.4+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% NC _(max)=0.6+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% N

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical section of the Fe—C—Cr—Mo—V—Nb—N system at 14 wt% Cr, 3.5 wt % Mo, 9 wt % V, 3.5 wt % Nb, and 0.13 wt % N;

FIG. 2 is a vertical section of the Fe—C—Cr—Mo—V—Nb—Co—N system at 14 wt% Cr, 3.5 wt % Mo, 9 wt % V, 3.5 wt % Nb, 2 wt % Co, and 0.13 wt % N;

FIG. 3 shows the etched microstructure (magnification of 1500×) of thealloy of the invention (04-099) hardened from 2150° F. in oil andtempered at 975° F. for 2h+2h+2h; and

FIG. 4 shows the etched microstructure (magnification of 1500×) of thehardened alloy (04-100) with no cobalt present.

DESCRIPTION OF THE EMBODIMENTS

Chemical Compositions Tested

Table 1 gives the chemical compositions that were examinedexperimentally and that led to the alloy of the article of the inventionthat achieves an improved combination of corrosion and wear resistantproperties. The reported alloys 03-192 through 04-099 are alloys inaccordance with the invention.

All examined compositions were prepared using the Crucible ParticleMetallurgy (CPM) technology. Prealloyed tool steel grades of the variousreported chemical compositions were melted in a nitrogen atmosphere,atomized by nitrogen gas, and hot-isostatically-pressed (HIP) at thetemperature of 2150° F. and the pressure of 14.5 ksi for four hours.

With respect to the various alloying elements in the wear and corrosionresistant tool steel are concerned, the following applies.

Carbon is present in an amount of at least 2.0%, while the maximumcontent of carbon may amount to 3.5%, and preferably in the range of2.7-3.0%. It is important to carefully control the amount of carbon inorder to obtain a desired combination of corrosion and wear resistance,as well as to avoid forming either ferrite or unduly large amounts ofretained austenite during heat treatment. The carbon in the articles ofthe invention may preferably be balanced with the chromium, molybdenum,vanadium, and nitrogen contents of the alloy of the invention accordingto the following formulae:C _(min)=0.4+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% NC _(max)=0.6+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% N

Nitrogen is present in an amount of 0.05-0.30%, and preferably in therange of 0.10-0.20%. The effects of nitrogen in the alloy of theinvention are rather similar to those of carbon. In tool steels, wherecarbon is always present, nitrogen forms carbonitrides with vanadium,niobium, tungsten, and molybdenum. Unlike carbon, nitrogen improves thecorrosion resistance of the alloy of the invention when dissolved in themartensitic matrix.

Silicon may be present in an amount of up to 1%, and preferably up to0.5%. Silicon functions to deoxidize the prealloyed materials during themelting phase of the gas-atomization process. In addition, siliconimproves the tempering response. Excessive amounts of silicon areundesirable, however, as it decreases toughness and promotes theformation of ferrite in the microstructure.

Manganese may be present in an amount of up to 1%, and preferably up to0.5%. Manganese functions to control the negative effects of sulfur onhot workability. This is achieved through the precipitation of manganesesulfide. In addition, manganese improves hardenability and increases thesolubility of nitrogen in the liquid prealloyed materials during themelting phase of the gas-atomization process. Excessive amounts ofmanganese are undesirable, however, as it can lead to the formation ofunduly large amounts of retained austenite during the heat treatment.

Chromium is present in an amount of 12.0-16.0%, and preferably in therange of 13.5-14.5%. The main purpose of chromium is to increase thecorrosion resistance, and, to a lesser degree, to increase hardenabilityand secondary-hardening response.

Molybdenum is present in an amount of 2.0-5.0%, and preferably in therange of 3.0-4.0%. Like chromium, molybdenum increases the corrosionresistance, hardenability, and secondary-hardening response of the alloyof the invention. Excessive amounts of molybdenum, however, reduce hotworkability.

Vanadium is present in an amount of 6.0-11.0%, and preferably in therange of 8.5-9.5%. Vanadium is critically important for increasing wearresistance. This is achieved through the formation of vanadium-rich MCtype primary carbonitrides.

Niobium is present in an amount of 2.0-6.0%, and preferably in the rangeof 3.0-4.0%. Every percent of niobium is equivalent to the amount ofvanadium calculated as follows:% V=(50.9/92.9)×% Nbwhere 50.9 and 92.9 are atomic weights of vanadium and niobium,respectively. Niobium and vanadium are equivalent elements when it comesto the wear resistance. However, these two elements do not have the sameeffect on the corrosion resistance. The presence of niobium decreasesthe solubility of chromium in MC primary carbides, i.e.,niobium-vanadium-rich MC primary carbides contain a smaller amount ofchromium compared to vanadium-rich MC primary carbides. This in turnincreases the amount of “free” chromium in the matrix, which in turnincreases the corrosion resistance.

To illustrate the effect of niobium on the alloy of the invention,Thermo-Calc software, coupled with TCFE3 steel thermodynamic database,was used to model two alloys that have the equivalent amount ofvanadium; one with niobium (Fe-2.8C-14Cr-3.5Mo-9V-3.5Nb-2Co-0.13N) andthe other one without niobium (Fe-2.8C-14Cr-3.5Mo-11V-2Co-0.13N). Thetwo alloys have the same vanadium equivalency (11% V). Thermodynamiccalculations were performed for the following two austenitizationtemperatures: 2050° F. and 2150° F. The results are given in Tables 2and 3. The amount of “free” chromium in the matrix is higher in thealloy that contains niobium. Based on thermodynamic calculations, it hasbeen discovered that the presence of niobium decreases the solubility ofchromium in MC primary carbides (see Table 3), which in turn results ina higher level of “free” chromium in the matrix.

Cobalt is present in an amount of 1.5-5.0%, and preferably in the rangeof 2.0-3.0% in order to prevent the undesirable presence of ferrite (α)in the heat-treated microstructure of the alloy of the invention. TABLE1 Chemical compositions that were experimentally examined as well asmodeled with Thermo-Calc software. Alloy C Cr Mo W V Nb Co N 02-354 1.6416.89 2.85 2.78 — 3.66 5.25 0.206 02-355 1.77 16.85 2.85 2.78 — 3.665.23 0.207 02-356 1.88 16.87 2.86 2.79 — 3.66 5.23 0.205 02-357 1.9017.00 2.91 2.69 — 3.68 5.34 0.183 02-358 2.14 17.05 2.92 2.69 — 3.685.34 0.182 02-359 2.33 17.08 2.92 2.70 — 3.68 5.35 0.182 03-192 2.6114.23 3.02 — 8.10 3.08 1.95 0.157 03-193 2.66 14.23 3.02 — 8.10 3.081.95 0.157 03-194 2.71 14.23 3.02 — 8.10 3.08 1.95 0.157 03-195 2.8114.23 3.02 — 8.10 3.08 1.95 0.157 03-199 2.49 14.20 2.97 — 7.78 3.131.99 0.115 03-200 2.59 14.20 2.97 — 7.78 3.13 1.99 0.115 03-201 2.6414.20 2.97 — 7.78 3.13 1.99 0.115 04-098 2.76 13.76 3.49 — 8.98 3.501.96 0.127 04-099 2.83 13.76 3.49 — 8.99 3.51 1.96 0.134 04-100 2.6813.89 3.35 — 9.03 3.42 — 0.125

TABLE 2 Chemical composition of austenitic matrix at 2050° F. and 2150°F.. Chemical Composition of Austenitic Matrix [wt. %] Alloy [° F.] C CrMo V Nb Co N Fe  9V—3.5Nb 2050 0.42 13.39 2.45 1.19 0.008 2.48 0.0042bal. 11V—0Nb 0.43 12.55 2.29 1.43 — 2.46 0.0024 bal.  9V—3.5Nb 2150 0.5513.95 2.60 1.45 0.012 2.45 0.0062 bal. 11V—0Nb 0.56 13.08 2.46 1.75 —2.42 0.0038 bal.

TABLE 3 Chemical composition of MC primary carbides at 2050° F. and2150° F.. Chemical Composition of MC Primary Carbides [at. %] Alloy [°F.] C Cr Mo V Nb Co N Fe  9V—3.5Nb 2050 43.19 5.12 3.62 36.41 9.120.0028 2.19 0.35 11V—0Nb 41.95 7.44 3.84 43.84 — 0.0036 2.18 0.75 9V—3.5Nb 2150 43.15 5.86 3.33 35.91 9.09 0.0039 2.16 0.49 11V—0Nb 41.828.44 3.49 43.06 — 0.0049 2.15 1.05

TABLE 4 Heat treatment response of alloys hardened from 2150° F. in oiland tempered for 2 h + 2 h + 2 h. Tempering Temperature Bar No. 500 750975 1000 1025 1050 1100 1200 04-098 59.4 59.7 62.5 60.7 59.7 58.3 53.146.7 04-099 60.1 60.7 63.5 61.4 60.7 58.6 53.3 47.4 04-100 49.3 51.854.2 51.9 50.8 49.0 47.0 40.2 S90V 58.5 60.5 60.5

TABLE 5 Pin-abrasion wear resistance of alloys. Austenitization HardnessPin Alloy Temperature Temper [HRC] Abrasion 04-098 2150° F. 500° F. 59.549.5 mg 975° F. 62.5 33.7 mg 04-099 2150° F. 500° F. 60.0 45.4 mg 975°F. 63.5 29.4 mg 04-100 2150° F. 500° F. 49.5 65.0 mg 975° F. 54.0 49.1mg CPM S90V 2150° F. 500° F. 59.0 52.0 mg 975° F. 61.5 37.3 mg Elmax1975° F. 500° F. 57.0 70.0 mg 975° F. M390 2100° F. 500° F. 58.0 62.0 mg975° F. X235 2100° F. 500° F. 986° F./1022° F. 59.5 52.5  

TABLE 6 Calculated matrix chemical compositions of corrosion and wearresistant tool steels. Chemical Composition of Austenitic Matrix [wt. %]Alloy [° F.] C Cr Mo V W Nb Co N PRE 440C 1900 0.43 11.57 0.06 — — — —0.065 12.81 10V-12 2100 0.49 11.22 0.82 1.69 — — — 0.003 13.97 S90V 21000.54 12.33 0.75 1.71 — — — 0.002 14.84 Elmax 2100 0.57 12.7 0.92 1.17 —— — 0.021 16.08 S30V 2000 0.46 10.92 1.71 1.03 — — — 0.005 16.65 X2352100 0.52 13.97 0.91 1.15 — 0.01 — 0.013 17.17 M390 2100 0.52 13.79 0.931.31 0.55 — — 0.025 18.16 MPL-1 2100 0.54 12.64 2.37 1.67 — — — 0.00420.52 S110V 2100 0.48 13.66 2.53 1.31 — 0.01 2.47 0.005 22.09

TABLE 7 Pitting potentials (E_(pit)) in 1% NaCl aqueous solution.E_(pit) [mV] vs. SCE Alloy PRE 500° F. 750° F. 975° F. 1025° F. 440C12.81 −140 −249 −355 −321 Anval 10V-12 13.97 9 38 −180 −138 CPM S90V14.84 59 −17 −176 −183 Elmax 16.08 213 243 −211 −216 CPM S30V 16.65 79−2 −240 −236 X235 17.17 97 138 −164 −282 M390 18.16 160 −121 −170 −179MPL-1 20.52 −72 15 −94 −100 04-099 22.09 403 272 −17 −71Microstructure

FIG. 3 shows the microstructure of an alloy of the invention (alloynumber 04-099). The alloy was hardened from 2150° F. in oil and temperedat 975° F. for 2h+2h+2h. After etching with Vilella's reagent for 90seconds, the total volume of primary carbides was measured to be 21.7percent, the standard deviation being 0.7 percent.

During the designing stage, the thermodynamics calculations performed onthe Fe-2.8C-14Cr-3.5Mo-9V-3.5Nb-0.13N alloy indicated the presence offerrite (α) when the alloy is austenitized at a temperature that isbelow 2156° F. (see FIG. 1). The γ+MC+M₇C₃ field needed to be expanded,or, in other words, the line that divides the γ+MC+M₇C₃ field and theα+γ+MC+M₇C₃ field needed to be shifted toward the left-hand side of thediagram in order to prevent the presence of ferrite in the heat-treatedmicrostructure.

Additional thermodynamic calculations indicated that the addition ofabout 2 wt % of cobalt would sufficiently extend the γ+MC+M₇C₃ field,eliminating the possibility of the ferrite presence in the hardenedcondition (see FIG. 2).

The first set of compositions examined experimentally was centeredaround the Fe—C-17Cr-2.5Mo-2.5W-3.5Nb-5Co-0.2N system (alloys 02-354through 02-359; see Table 1). The problem with this alloying system wasretained austenite that was difficult to transform into martensite evenafter sub-zero treatments.

The second set of compositions examined experimentally was centeredaround the Fe—C-14Cr-3Mo-8V-3Nb-2Co—N system (alloys 03-192 through03-195 and 03-199 through 03-201). The levels of carbon balance testedranged from −0.20 to +0.20, and were calculated using the followingformula:C _(bal)=% C−[0.4+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×%N]

It is a well established fact that the amount of carbon present in thesteel has the most profound effect on the properties of any corrosionand wear resistant tool steel grade. The amount of carbon has a directeffect on the hardness, the wear resistance, and the corrosionresistance of wear and corrosion resistant tool steel. For a givenchemical composition of the steel, the carbon balances were targeted tobe close to zero (±0.2%).

The alloys that are based on the Fe—C-14Cr-3Mo-8V-3Nb-2Co—N systemexhibited better hardness response, better corrosion resistance, andmarginally better wear characteristics when compared to other corrosionand wear resistant martensitic tool steels.

In order to examine whether the wear and corrosion resistance of thesecond set of compositions could be further improved, an additional setof compositions centered around theFe-2.8C-14Cr-3.5Mo-9V-3.5Nb-2Co-0.13N system was manufactured andexperimentally examined (alloys 04-098 through 04-100). The tests showedthat the alloys of the third set exhibited better heat treat response(see Table 4) and better wear characteristics (see Table 5) compared toCPM S90V. The alloy of the invention also has better corrosionresistance (see Table 6) compared to other widely used corrosion andwear resistant tool steels (see Table 6).

Influence of Cobalt on the Microstructure

An alloy (04-100) was prepared specifically to demonstrate the influenceof cobalt and the necessity to use it in the alloy of the invention.Both thermodynamic calculations and experimental results clearlyindicated that the Fe-2.8C-14Cr-3.5Mo-9V-3.5Nb-0.13N system has tocontain at least 1.5 wt. pct. Co, if ferrite is to be eliminated fromthe heat treated microstructure. The major alloying elements in thealloy of the invention (chromium, molybdenum, vanadium, and niobium) areall ferrite forming elements. The presence of ferrite, as well as a poorheat treat response, was indeed observed in the alloy that contained nocobalt (04-100).

As predicted by thermodynamic calculations, the matrix of the heattreated alloy that contains no cobalt (alloy number 04-100) has someferrite present (see FIG. 4), which resulted in poor heat-treat responsefor the alloy (less than 54 HRC). The other two alloys of the third setthat contain about 2 wt. pct. of cobalt (04-098 and 04-099) developeddesired heat-treated responses (62.5 HRC and 63.5 HRC, respectively) aswell as microstructures that consist of V—Nb-rich MC and Cr-rich M₇C₃primary carbides in the matrix of tempered martensite.

Corrosion Resistance

Pitting Resistance Equivalent Number: The pitting resistance equivalentnumber (PRE) is useful for evaluating the resistance of an austeniticstainless steels to pitting and crevice corrosion. The PRE is calculatedusing the following equation:Cr+3.3(Mo+0.5W)+16N

Generally, the PRE is calculated using the bulk chemical composition.However, the alloys disclosed herein contain high amounts of primarycarbides that deplete the matrix of some of the necessary elementsneeded for corrosion resistance. Therefore, the PRE of these alloys wascalculated using an estimated matrix composition as determined byThermo-Calc software (see Table 6). The alloys are listed by increasingPRE values.

Based on the matrix composition, the invention alloy (04-099) has thehighest PRE even though it does not have the highest matrix chromiumcontent. The PRE of this alloy (04-099) is even higher than those alloyswith higher bulk chromium contents such as MPL-1, X235, M390 and Elmax.Since the matrix chromium content of these alloys is similar, the highPRE of the invention alloy is due to its high contents of chromium andmolybdenum in the matrix. This is because 30-47.5% of the chromium inthe high chromium alloys is used in the formation of the primarycarbides in these materials. Only about 2.5% of the chromium in theinvention alloy is used in the formation of the primary carbides therebykeeping most of the chromium in the matrix to aide in corrosionresistance. More chromium is present in the matrix in the inventionalloy due to the presence of niobium and vanadium which preferentiallyform more stable MC type carbides compared to the M₇C₃ type (chromiumrich) carbides.

Corrosion Tests: Potentiodynamic tests were used to evaluate the pittingresistance of the invention alloy and of commercially available wear andcorrosion resistant alloys. The tests were conducted in an aqueoussolution containing 1% NaCl. The tests were conducted by varying thepotential from −0.8V vs. SCE (saturated calomel reference electrode) toat most 0.5V at a scan rate of 0.2 mV/sec. Two graphite rods were usedas the counter electrodes. The test solution was purged with nitrogengas for at least 20 minutes before testing each specimen. The pittingresistance of the alloys is defined by the pitting potential (E_(pit))obtained from a potentiodynamic curve. The more positive the pittingpotential, the more resistant the alloy is to pitting. Prior to eachtest, the specimen was ground down by 600 grit paper. The specimen wasthen washed and dried with alcohol.

Depending on the application, the wear and corrosion resistant alloysare given different heat treatments. If corrosion resistance is ofutmost concern, the alloy is typically tempered at or below 750° F.,which allows more of the chromium to stay in the matrix by minimizingthe precipitation of secondary carbides. If hardness and wear resistanceis the primary concern, then the alloys are typically tempered at 950°F. and above to allow for secondary hardening effects to take place.Therefore, each alloy was tempered at 500° F., 750° F., 975° F. and1025° F.

Corrosion Resistance Results: The pitting potential (E_(pit)) for eachalloy at each tempering temperature is given in Table 7. The resultsshow that the invention alloy (04-099) with the highest PRE also has thehighest resistance to pitting at all tempering temperatures. The E_(pit)for the invention alloy is almost 50% higher that that of the nextclosest alloy, Elmax, at a tempering temperature of 500° F. In general,the alloys with 18-20% bulk chromium content, i.e., Elmax, M390 andX235, have mediocre pitting resistance compared to the invention alloyat all tempering temperatures. The alloy with the highest bulk chromiumcontent actually has one of the lowest pitting potentials at the lowtempering temperatures. These results indicate that the total chromiumcontent is not an indicator of how resistant the material is tocorrosion.

The matrix compositions of X235 and the alloy of the invention aresimilar. However, the pitting resistance of these two alloys issignificantly different. This difference in pitting resistance isattributed to the higher molybdenum content of the invention alloy. Thecobalt in the invention alloy is not expected to significantly affectthe pitting resistance of the alloy of the invention.

Heat Treatment Response

When compared with CPM S90V, the alloys of the invention (04-098 and04-099) offer better heat-treatment response—approximately 1.5-2.0 HRChigher for the same heat treatment. The heat-treatment responses of thealloys of the invention and CPM S90V are given in Table 4.

Abrasive Wear Resistance

All the pin-abrasion wear resistance test specimens were austenitized at2150° F. for 10 minutes, quenched in oil, and then tempered at either500° F. (for maximum corrosion resistance) or 975° F. (for maximumsecondary-hardening response) for 2h+2h+2h. The results are given inTable 5. The pin-abrasion wear resistance of other corrosion and wearresistant martensitic tool steels is included as well for comparisonpurposes.

All element amounts are reported in weight percent.

1. A corrosion and wear resistant tool steel article produced by hotisostatic compaction of nitrogen gas atomized prealloyed powderparticles consisting essentially of, in weight percent: C: 2.0-3.5; Si:1.0 max.; Mn: 1.0 max.; Cr: 12.0-16.0; Mo: 2.0-5.0; V: 6.0-11.0; Nb:2.0-6.0; Co: 1.5-5.0; N: 0.05-0.30; and the balance is essentially ironand incidental impurities.
 2. A corrosion and wear resistant tool steelarticle produced by hot isostatic compaction of nitrogen gas atomizedprealloyed powder particles consisting essentially of, in weightpercent: C: 2.7-3.0; Si: 0.50 max.; Mn: 0.50 max.; Cr: 13.5-14.5; Mo:3.0-4.0; V: 8.5-9.5; Nb: 3.0-4.0; Co: 2.0-3.0; N: 0.10-0.20; and thebalance is essentially iron and incidental impurities.
 3. The article ofclaim 1 or claim 2, wherein carbon is balanced with chromium,molybdenum, vanadium, and nitrogen in accordance with:C _(min)=0.4+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% NC _(max)=0.6+0.099×(% Cr−11)+0.063×% Mo+0.177×% V+0.13×% Nb−0.85×% N.